Thermodynamics of anisotropic surfactant micelles.: I.: The influence of curvature free energy on the micellar size and shape

A novel theory for the structural behavior of surfactant micelles is expounded. The micelles are considered to be generally shaped as triaxial tablets with distinct thickness width and length, respectively, and may become spherical, spherocylindrical or disk-shaped in the special cases where two or more of the dimensions are equal. It is demonstrated that the average width and length of a tablet-shaped micelle with a fixed thickness is mainly determined by two constants, k(1) and k(2), related to the first and second order correction in curvature of the micellar end caps. The size of the micelles is found to be mainly determined by k(1), whereas k(2) influences the shape, i.e., the length-to-width ratio, of the micelles so that the micellar size increases with increasing k(1) and the length-to-width ratio decreases with increasing k(2). Hence, large positive values of k(2) promote the formation of tablets rather than very long spherocylinders. An additional parameter related to the curvature of the straight cylindrical rims may influence the structure of the tablet-shaped micelles insofar k(2) is close to or below zero. (C) 2000 American Institute of Physics. [S0021-9606(00)50737-5].